The Hiriidinea of Illinois. 533 



almost exactly the same in several species, which are never- 

 theless readily distinguished by other and very obvious char- 

 acters, and especially by the terminal portion of the male 

 genital ducts, 



Erpohdella punctata reaches a large size and has a more 

 robust form than any other of our common nephelids. In 

 preserved specimens the dorsal surface is rougher and the 

 posterior lateral margins thinner than in most other species. 

 The body is very muscular and has a firm feel. 



For the purpose of comparison with the species of Dina 

 which are described below some features of the complete 

 somites may be mentioned. Of these there are seventeen, 

 YIII to XXIV inclusive. All of the live annuli are of 

 nearly equal size, but b 6 is frequently slightly enlarged. 

 This is, however, never very obvious, and the annulus is 

 never marked by a transverse furrow except in strongly con- 

 tracted specimens in which all of the annuli are equally 

 affected. .12 is noteworthy as being rougher than the 

 secondary annuli, and its papillie are frequently larger and 

 more numerous, extend further marginally, and are more 

 subject to concrescence into a transverse ridge. When fully 

 extended all of the annuli are free from cross-furrows or 

 wrinkles, but when contracted irregular and interrupted 

 transverse furrows may appear on all of the annuli. These 

 may occur anterior to the papilla only, or both anterior and 

 posterior, in which latter case the annulus is more or less 

 completely divided into three parts, of which the middle bears 

 the principal papilla'. Such transient subdivisions must not 

 be mistaken for the true tertiary annuli which appear in 

 Dina, etc. 



Reproductive Organs. — The external male organ when fully 

 extruded has the form of a low circular disc occupying nearly 

 the entire median width of two annuli. It consists of a 

 marginal rim fitting closely around a transverse elliptical 

 central cushion of al)out twice the height of the rim. The 

 terminal openings of the sperm-ducts appear well separated 

 on the sides of the cushion. 



The testes extend through six and one half somites 



